1. Field of the Invention
The subject invention pertains to the preparation of fiber reinforced and non-fiber reinforced thermoplastic honeycomb. More particularly, the subject invention pertains to processes by means of which such honeycomb may be produced in an economical, continuous manner.
2. Description of the Related Art
Paper honeycomb was first made by the Chinese approximately two thousand years ago, but at that time it was used primarily as ornamentation and not as a structural material. The modern utilization of honeycomb structures began just after 1940, and today there are about ten companies manufacturing the various core types.
While the primary utilization of honeycomb structure is in the construction of sandwich panels, it has many other applications, such as energy absorption, air directionalization, light diffusion, and radio frequency shielding.
U.S. Pat. No. 4,500,583, to Naul, discloses a honeycomb structure made of resin impregnated molded glass wool. In particular, glass wool blankets containing about 20 to 25 percent by weight of an uncured binder such as urea-phenol-formaldehyde resin are molded into corrugated sheets under heat and pressure in a two-part mold. A plurality of the corrugated sheets can then be adhesively bonded to one another to form a honeycomb structure.
U.S. Pat. No. 2,734,843, to Steele, discloses a method of producing honeycomb wherein longitudinally extending, spaced, parallel lines of adhesive are applied to the face surface of continuously moving web material, the web material is cut into separate flat sheets of uniform size, and said sheets are adhered to one another with the obverse side of each sheet adhered to an adjacent sheet by a plurality of spaced parallel lines of adhesive and the reverse side of each sheet adhered to an adjacent sheet by a plurality of spaced parallel lines of adhesive, which are in staggered parallel relationship to the lines of adhesive on the obverse side.
U.S. Pat. No. 3,032,458, to Daponte et al., discloses a method of making an expandable structural honeycomb material which comprises securing together a number of layers of flexible sheet material in a stock by means of an adhesive distributed between the layers in patches arranged in arrays of intersecting rows and columns and positioned such that the columns at the obverse face of each intermediate layer are staggered with respect to the columns at the reverse face of said layer while the rows at said faces are coincident, and slicing the stack by cutting it in the direction of the rows at position such that the contacting pairs of faces of the sheet material within the slices thus producing are secured together over a part only of their width by at least a part of a single row of patches.
U.S. Pat. No. 3,028,481 to Covert discloses a method of forming metal honeycomb materials whereby a strip of uncorrugated metal is corrugated as part of an integrated honeycomb welding operation. The honeycomb is welded by means of crescent shaped electrodes having a circumference substantially equal to or greater than the honeycomb thickness.
U.S. Pat. No. 3,225,433 discloses one method of aligning corrugated metal webs in a node-antinode relationship to eliminate problems associated with stretching and/or distortion of preformed corrugated webs.
U.S. Pat. No. 4,128,678, to Metcalfe et al., discloses a method and apparatus for the manufacture of a heat insulating material from an unsecured, strip-shaped felt of fibers containing a heat hardenable bonding substance. The felt is first formed into a serpentine array of corrugations extending across the entire width of the uncured felt and then cured. The felt is then cured and the cured felt is cut longitudinally into two partial felts, the corrugations being severed so as to form a succession of U-shaped arrays along each of the partial felts.
U.S. Statutory Invention Registration H47, to Monib, discloses a lightweight structural panel of an aramid honeycomb core faced with a resin-impregnated fiber layer, wherein peel strength between the core surface and the facing layer is improved by interpositioning of a spunlaced fabric, containing at least 50% aramid fibers pervaded with a curable resin, between the core surface and the facing layer.
U.S. Pat. No. 4,012,738, to Wright, discloses a microwave radiation absorber comprising a layer of dielectric material of relatively high dielectric constant and a layer of magnetic material having a relatively high coefficient of magnetic permeability.
U.S. Pat. No. 3,600,249, to Jackson et al., discloses a method and apparatus for the production of a reinforced plastic honeycomb comprising the steps of: (1) impregnating a fabric which distorts under its own weight, such as a fiber glass fabric, with a heat curable resin in an amount sufficient to cause the fiber glass fabric to have sufficient body to prevent its distorting under its own weight while permitting expansion after curing; (2) applying adhesive lines on the impregnated fiber glass fabric, the adhesive being applied so as to avoid penetration to the opposite side of the fiber glass fabric, and allowing said adhesive lines to advance to a relatively non-tacky state; (3) stacking sheets of the so-produced fiber glass fabric with the lines of adhesive on one sheet in staggered relation to the lines of adhesive on adjacent sheets; (4) applying heat and pressure to the so-formed stack to cause the adhesive to flow and bond to the surface of the next adjacent sheet in the stack; (5) expanding the stack to form a honeycomb configuration; (6) applying heat and pressure to the expanded stack created in step (5) to fully cure the impregnated resin and the adhesive; (7) dipping the rigid honeycomb structure formed in step (6) into a mass of uncured resin; and ( 8) following the dipping, curing the resin so-coated onto the rigid honeycomb structure.
U.S. Pat. No. 3,321,355, to Holland, discloses a method of making a honeycomb structure from fabric reinforced plastic wherein the warp and woof of the fabric in the final honeycomb product are obliquely disposed to the longitudinal axis of the honeycomb cells. In particular, the method comprises: providing a plurality of non-rectangular parallelogram shaped cut sections of fabric reinforced plastic material of substantially the same pattern and size in which the warp of the fabric extends parallel and the woof of the fabric extends perpendicular to a first pair of parallel sides of each section and at acute angles in reference to a second pair of parallel sides of each section; superimposing such sections one upon the other in a stack; adhering such sections to one another along spaced apart parallel bonding lines extending perpendicular to said second pair of parallel sides, the bonding lines of successive superimposed sections staggered relative to one another to form a honeycomb structure.
U.S. Pat. No. 3,598,676, to Noble, is an improvement over the aforementioned Holland patent, to reduce waste material, i.e., to eliminate the step of trimming portions of the parallelogram shaped core to produce a rectangular shaped core. In particular, the improved method comprises: forming a plurality of non-rectangular parallel-ogram shaped sections of fabric reinforced plastic material in which a first and second side of the section are substantially parallel to each other and to the warp or woof of the fabric and in which a third and fourth side of the section are substantially parallel to each other and disposed at an oblique angle to the warp and woof of said fabric, the distance between the third and fourth sides of each section being substantially equal; joining the first and second sides of said sections together in serial relationship to form a web having a width equal to the distance between the third and fourth sides of one of said sections and a length approximately equal to the sum of the first sides of all sections which are joined together in serial relationship, the third and fourth sides of said joined sections forming the lateral edges of the web; cutting a plurality of equal rectangular shaped sections from said web, two sides of each rectangular section being cut perpendicular to the lateral edges of the web; superimposing a plurality of said rectangular sections one upon another in a stack; and adhering said plurality of rectangular sections to one another along spaced apart bonding lines which are substantially parallel to each other and perpendicular to two sides of said superimposed rectangular sections, the bonding lines of adjacent superimposed sections being staggered relative to one another to form a plurality of adjacent cells having longitudinal axes which are substantially parallel to each other and perpendicular to two sides of said superimposed rectangular sections, whereby a bias weave honeycomb core structure is formed in which the warp and the woof of said fabric are disposed at an oblique angle to the longitudinal axes of said cells.
U.S. Pat. No. 3,759,775, to Shepherd, discloses a method for producing an absorbent, high bulk, very low fiber density stabilized web. In particular, an air laid web of fibers is thoroughly impregnated with a volatile liquid. The volatile liquid may contain a small amount of heat-activatable binder, or the web of fibers may include the binder in the form of a small amount of thermoplastic fibers or powder dispersed throughout the web. The so-impregnated web is then heated, preferably, by dielectric heating or the like, so as to vaporize the liquid whereby the web is explosively puffed up and the small amount of binder secures interconnections of the fibers to maintain the web superstructure.
U.S. Pat. No. 3,134,705 discloses the preparation of flanged, non-reinforced honeycomb materials by fusing together corrugated thermoplastic webs by means of a pair of heat sealing jaws one node-antinode at a time. The resulting honeycomb is collapsed for shipment, reexpanded, and flanged by placing between heated platens.
U.S. Pat. No. 3,366,525, to Jackson, discloses a method of making honeycomb or similar laminated structures from sheets of heat sealable plastic which are cocured together under heat and pressure at selected areas. In an example a polyethylene web 10 inches wide and 4 mils thick is cut into sheets of material 18 inches long. A release film is printed on the sheets in lines 0.441 inch wide which are spaced apart by exposed or release-film-free regions 0.135 inch wide. The sheets are stacked in a mold and then subjected to heat and pressure to seal adjacent sheets together in those regions free of release film. The mold is cooled, the pressure reduced, and the stack of heat sealed sheets removed from the mold. The stack is then heated and pulled to expanded condition and the so-formed honeycomb is then cooled.
As may be readily ascertained from the above-noted documents, the preparation of honeycomb structure from fiber-reinforced plastics requires numerous web handling steps including multiple impregnation and/or dipping steps. In the case of obtaining higher shear modulus and improved handleability, the cutting of woven webs on the bias and their re-orientation requires even more handling steps.
U.S. Pat. No. 3,356,555 to Jackson discloses a batch method of preparing honeycomb core wherein a squirrel cage roller containing hexagonal bars operates in conjunction with a plurality of hexagonal major bars on a fixed bed to provide a corrugated web by the action of compressing a heated thermoplastic web between the advancing squirrel cage bars and the fixed bed bars. The corrugated webs thus produced are then assembled into a honeycomb structure by placing a first corrugated web on the major bars of a bed followed by placing minor bars in the nodes above the web. A second corrugated sheet is then located with its nodes adjacent to the first web's antinodes. An additional layer of minor bars is placed on top and the procedure repeated until the desired honeycomb thickness is achieved. The assembly is then placed between caul plates and platens and heated to form a finished honeycomb product.
In GB-A-2 188 866, a batchwise method of preparing thermoplastic honeycomb is disclosed wherein shaped formers as disclosed by Jackson in U.S. Pat. No. 3,356,555 are disposed between adjacent corrugated thermoplastic sheets, the assembly pressurized, and heated to cause the thermoplastic to fuse. Following the preparation of one multiple cell honeycomb segment by such batchwise operation, the section may be advanced to allow a further section to be laid up and fused. The process described requires pressurizing and heating the entire newly formed honeycomb structure.